1. Field of the Invention
This invention relates to a large-size pneumatic radial tire for use on rough road, and more particularly to a large-size pneumatic radial tire having improved durability in the running on rough road by using a steel cord having a particular shape and a sealing-type rubber penetration form as an outermost cord layer for belt.
2. Description of the Prior Art
Heavy duty pneumatic radial tires for trucks and buses using steel cords as a reinforcement for the belt and the carcass are used not only in the running on good roads inclusive of completely paved and managed road but also in the running on rough roads having a poor road surface state such as a construction road or the like. Particularly, in case of the running on roads inclusive of rough roads, there are frequently caused problems of separation failure between tread and belt (hereinafter abbreviated as TLB), i.e. abnormal drop of tire use life due to peeling break of tread rubber layer from belt at an initial use stage of tire, impossibility of recapping tread rubber layer due to peeling and cut failure of tread accompanied with extremely poor appearance at a last use life stage, and the like.
As a cause of generating such a TLB, it is thought that water begins to penetrate form the exterior into the inside of the cord together with the cord breaking up of the belt due to cut failure or the like of the tread and then the penetrating water easily progresses in the inside of the cord along the cord direction because the penetrability of rubber into the inside of the conventional cord is poor, whereby the interface between rubber and cord is degraded by water to cause TLB. As a countermeasure against TLB, it has hitherto been attempted to use a high extensible cord of a strand construction as an outermost cord layer for the belt in order to reduce cord cut among the causes producing TLB. The use of such a strand construction cord certainly reduces the cord cut and also protects a cord layer beneath the outermost cord layer because of a large extension. However, this cord is poor in rubber penetrability into the inside of the strand, so that after the cut reaches to the cord, water easily progresses in the inside of the cord independently of the presence or absence of the cut of the cord itself to cause TLB. Therefore, the problem of TLB has not satisfactorily been solved by the use of the strand construction cord. Moreover, in the above high extensible cord, the weight per cord length is large, but the tensile strength is small and also the cost per unit weight of cord is expensive.